(1) Field of the Invention
The invention is related to an electric circuit with one power amplifier for two piezoelectric actuators and said invention particularly belongs to the field of power electronics for piezoelectric actuators, in particular for helicopters.
(2) Description of Related Art
Piezoelectric actuators have the advantage of a high actuating precision and a fast reaction. Piezoelectric actuators are components with a high electrical capacity whereby only part of the electrical energy supplied to the actuators is converted to mechanical energy. A large part of this energy is stored in the piezoelectric actuator functioning as a capacitor.
In a dynamic, repetitious or continuous operation of a piezoelectric actuator, considerable electrical power in the form of apparent power flows through the actuator. This apparent electrical power has to be supplied by the driver circuit of the actuator. In the case of periodic driving or energizing the piezoelectric actuator is electrically alternately charged and discharged, whereby electrical energy is cyclically supplied to and withdrawn from the piezoelectric actuator. In known driver circuits for periodic or repetitious driving of the piezoelectric actuator, the stored electrical energy is dissipated during the discharge phase or cycle in the driver or control circuit which has for example an ohmic resistance for the dissipation.
The document US 2008/0316671 A1 discloses a piezoelectric actuator with an amplifier for a piezo arrangement based on pulse-width modulation (PWM) using switches SW in combination with inductors L. The inductors are used as low pass filters to buffer energy at high frequency of PWM, e.g. in the order of 20 kHz. A detailed PWM generation scheme is disclosed with independent charging and discharging circuits. Only unipolar operation is possible.
The document US 2005/0285477 A1 discloses two piezo-elements which are driven with two voltages having a phase difference. One of the piezo-elements is charged by an inductive step-up means to a voltage exceeding an available power supply voltage. Subsequent charge, i.e. energy is transferred from the one piezo-element to the other piezo-element by an inductive element. Only unipolar operation is possible.
The document US 2005/0280332 A1 discloses two piezo-elements of a piezoelectric actuator which are driven with two voltages having a mutual phase difference. One of said piezo-elements is charged by a capacitive voltage step-up means to a voltage exceeding an available power supply voltage. A capacitor is coupled to the piezo-element, which divides the voltage of the piezo-element and simultaneously transfers part of the energy stored in the piezo-element to said capacitor. A part of the energy transferred to said capacitor is later transferred back to said piezo-element, providing an energy-saving feature. The second piezo-element is driven in a similar way providing the required phase difference. According to US 2005/0285477 A1 and US 2005/0280332 A1 there are respectively 2 amplifiers for 2 piezo-elements.
The document U.S. Pat. No. 6,411,009 A discloses a piezoelectric actuator with at least two piezoelectric actuator elements connected in series with each other mechanically and electrically. Said two piezoelectric actuator elements form a half bridge in an amplifier bridge circuit having a further half bridge formed by two series connected electronic switches which are operated or clocked by a control circuit such as a pulse modulator circuit for periodically energizing the piezoelectric actuators in push-pull fashion.
A choke is connected between a first junction point of the two piezoelectric actuators and a second junction point between the two electronic switches for assuring a loss free reverse charging of the two piezoelectric actuators functioning as electrical capacitors in the energizing bridge circuit. The choke functions as energy storage and the stored energy is used in the push-pull charging of the capacitors formed by the piezoelectric elements. The choke is used as low-pass filter to suppress the oscillations of a pulse width modulator generated by two switches driving the two piezoelectric actuator elements. The piezoelectric actuator according to U.S. Pat. No. 6,411,009 A allows only unipolar operation.